Cyclododecane conformation

Fig. 1. 13C-NMR fast exchange-slow exchange transition for the conformational interconversion of cyclododecane in solution of propane-d, (left side) and in the solid by CP-MAS techniques (right side) at 75.47 MHz. The temperature decreases from top to bottom as indicated at the spectra. Chemical shifts are given at the signals and refer to TMS = 0 ppm. (Ref.7))...

Fig. 7. Molecular model of cyclododecane in the (gag)4 conformation of the crystalline state according to Dunitz and Shearer (Ref. 12>). The numbers at the bonds indicate the rotational angles...

As we have seen, the anomeric effect confers a double-bond character to each C—0 bond of conformer D the energy barrier for a C —0 bond rotation in acetals must therefore be higher than that observed in simple alkanes. Borgen and Dale (41) may have provided the first evidence for this point by observing that 1,3,7,9-tetraoxacyclododecane (37) has a much higher conformational barrier (11 kcal/mol) than comparable 12-membered rings such as cyclododecane (7.3 kcal/mol (42) or 1,4,7,10-tetraoxacyclododecane (5.5 and 6.8 kcal/mol (43)). It was also shown that the two 1,3-dioxa groupings in 37 exist in a conformation identical to that of dimethoxymethane, i.e. the conformation D. 

Weiner et al. (32) have described the application of distance geometry to the conformational analysis of cyclooctane, cyclododecane, 18-crown-6, and... 

The most important structural features that influence the conformation and reactivity of cycloalkanes differ depending on whether small (cyclopropane and cyclobutane), common (cyclopentane, cyclohexane, and cycloheptane), medium (cyclooctane through cycloundecane), or large (cyclododecane and up) rings are... 

The first molecule that shows a disordering transition different from the plastic crystals of Sect. 3.1.1 is cyclododecane, (C,2H24). Only a minor transition is fouiMi at 199K . Infraredand X-ray analysis permit the establishment of the ring structure as shown in Fig. 3.4. Also shown in Fig. 3.4 are the hiMR spectra for the three condensed phases . At 150 K two different carbon atoms can be identified. One bordered by tg and gt bond conformations, representing the... 

A schematic perspective drawing 53 is particularly useful, with a view from the side and somewhat above the plane, to avoid overlap of atoms. This is usually a version of the true perspective drawing that a computer might produce, modified to clarify significant aspects of the conformation. Models help. The diagrams 51-53 illustrate the preferred conformation of cyclododecane in these conventions... 

Beyond nine-membered rings it is decreasingly helpful to use words like boat and chair to describe conformations. The investigation of the conformation of cyclododecane by various techniques has been summarized recently . The highly symmetrical [3333] conformational description applies to two quite different conformational minima of widely different energy, indicating a shortcoming of the numerical nomenclature. 

From the entropies of disordering in Fig. 5.141, one notes that cyclododecane gains almost no disorder in the low-temperature transition, while cyclotetraeicosane may develop 11 to 18 conformational isomers [calculated from 127.7/(7 to 12), based on Fig. 2.103]. The conclusion reached is that cyclododecane may undergo rotoreptation, a rotation with simultaneous change of conformational isomers. This motion leads to little disorder, particularly in a jump-hke motion. Schematically, this jump process is shown in Fig. 5.142 for cyclodoeicosane. One can treat this rotoreptation similat to the jump-rotation, seen in cyclopropane and benzene [46]. 

The conformation state of cyclononane is a crown with a symmetry number of 1 but it is insirfficiently studied and for this reason we have combined the calculations by the Alinjer method with the X-ray data for cyclononylamine bromohydrate [80]. The conformation of cyclododecy-lamine-1,6 dihydrohydrate, which has only one axis of symmetry of second order, is taken as a base for cyclodecane. According to the X-ray data cyclododecane has a square shape conformation with butane segments as... 

Deuterium isotope shifts over up to six bonds have been observed in the nmr spectrum of deuteriated cyclodecanones (Wehrli et al., 1978). The equilibrational origin of the observed long range effects was briefly discussed by Anet and Dekmezian (1979) and was explained in detail by Whipple et al. (1981). In the low temperature spectra of deuteriated cyclododecane iso-topomers Anet and Rawdah (1978) have also observed deuterium isotope effects which are likely to have a conformational origin and to arise from the lack of precise D4-symmetry in the preferred conformation of that hydrocarbon. 

The extent of the restriction on the mobility of an azobenzene moiety covalently incorporated into higher-order unimer micelles, monitored by photoisomerization and thermal back isomerization rates, is much more pronounced in cyclododecyl domains than in dodecyl domains, which can be related to a difference in the mobility of these aliphatic hydrophobes in the microdomains (52). Cyclododecane is a rigid molecule with much less conformational freedom than dodecane because of its cyclic structure. The latter is a flexible chain that can adopt a variety of conformations. Therefore, cyclododecyl groups may be more tightly associated than dodecyl groups and form a more rigid hydrophobic microdomain. 

Make molecular models of cyclopropane through cyclododecane. Compare the relative conformational flexibility of each ring with that of others within the series and with that of the corresponding straight-chain alkanes. 

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